Self-assembly of coatings utilizing surface charge

ABSTRACT

An apparatus and method for measuring the isoelectric pH for materials deposited on or otherwise affixed onto and in contact with an electrode surface, and a method for utilizing the isoelectric pH to form nanometer thickness, self-assembled layers on the material, are described. Forming such layers utilizing information obtained about the isoelectric pH values of the substrate and the coating is advantageous since the growth of the coating is self-limiting because once the surface charge has been neutralized there is no longer a driving force for the solid electrolyte coating thickness to increase, and uniform coatings without pinhole defects will be produced because a local driving force for assembly will exist if any bare electrode material is exposed to the solution. The present self-assembly procedure, when combined with electrodeposition, may be used to increase the coating thickness. Self-assembly, with or without additional electrodeposition, allows intimate contact between the anode, electrolyte and cathode which is required for successful application to solid-state batteries, as an example.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 61/318,068 for “Isoelectric Determination Apparatus andUse In The Fabrication Of Batteries” by Amy L. Prieto et al., which wasfiled on Mar. 26, 2010, the entire contents of which is herebyspecifically incorporated by reference herein for all that it disclosesand teaches.

BACKGROUND OF THE INVENTION

Secondary lithium-ion batteries have found multiple applications inportable electronics where high charge and discharge rates are notrequired to improve performance. However, high rates become importantwhen considering the use of rechargeable lithium-ion batteries in thetransportation industry. Electrode materials having irregular surfacesresulting in high interfacial surface areas and short characteristicdiffusion lengths are expected to provide batteries with high powerdensities. Producing uniform, defect-free surface coatings for suchelectrodes for lithium-ion batteries having electrically insulating, butionically conducting electrolytic separator materials on the nanoscale(for either the negative and/or positive electrodes), has proved to bedifficult.

SUMMARY OF THE INVENTION

Self-assembly of surface coatings using electrostatic forces has notbeen widely pursued for high surface area structures in the past becauseof the difficulty in determining the isoelectric pH; that is, the pH atwhich there is no net surface charge.

Embodiments of the present invention overcome the disadvantages andlimitations of the prior art by providing an apparatus and method formeasuring the isoelectric pH of materials that can be deposited orotherwise affixed on and in electrical contact with an electrodesurface.

It is further an object of embodiments of the present invention toprovide a method utilizing the isoelectric pH of a material to form aself-assembled layer having nanometer thickness.

Another object of embodiments of the present invention is to provide amethod for increasing the thickness of the self-assembled layer.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the apparatus for determining the isoelectric pH of a material,hereof, includes: a first chamber adapted for containing a solutionhaving a chosen pH including: a circular disk having an axis, a firstface, and an opposing second face onto which the material is disposed; ashaft attached to the first face of the disk; a first referenceelectrode disposed in the vicinity of the second face of the disk and inthe vicinity of the axis; and a hollow tube affixed to a wall of thefirst chamber at an orifice therein; a second chamber in fluidcommunication with the first chamber through the tube, including asecond reference electrode; means for rotating the shaft at a chosenrate; and means for measuring the potential difference between the firstreference electrode and the second reference electrode.

In another aspect of the present invention and in accordance with itsobjects and purposes, the method for determining the isoelectric pH of amaterial, hereof, includes: affixing the material onto one face of acircular disk having an axis of rotation and disposed in a solutionhaving a selected pH value in a first chamber; rotating the disk at achosen rate; measuring the potential difference between a firstreference electrode disposed in the vicinity of both the axis of thedisk and the material and a second reference electrode disposed in asecond chamber in fluid communication through a tube with said firstchamber; adjusting the pH of the solution such that the potentialdifference is approximately equal to the resting potential differencebetween the first reference electrode and the second referenceelectrode; and measuring the pH of the solution when the potentialdifference is approximately equal to the resting potential difference.

In yet another aspect of the present invention and in accordance withits objects and purposes, the method for depositing a coating materialonto a substrate, hereof, includes: determining the isoelectric pH ofthe substrate; determining the isoelectric pH of the coating material;preparing a solution of the coating material having a pH between theisoelectric pH of the substrate and the isoelectric pH of the coatingmaterial; and immersing the substrate into the solution for a timesufficient for self-assembly of the coating material onto the substrate.

Benefits and advantages of embodiments of the present invention include,but are not limited to, providing a method for measuring the isoelectricpH of materials whereby the pH may be determined for which theelectrostatic formation of a solid electrolyte coating on a substrate isadvantageous since the growth of the coating is self-limiting becauseonce the surface charge has been neutralized there is no longer adriving force for the solid electrolyte coating thickness to increase,and since uniform coatings without pinhole defects will be producedbecause a local driving force for assembly will exist if any bareelectrode material is exposed to the solution. The present self-assemblyprocedure, when combined with electrodeposition, may be used to increasethe coating thickness. Self-assembly, with or without additionalelectrodeposition, allows intimate contact between the anode,electrolyte and cathode which is required for successful application tosolid-state batteries, as an example.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a schematic representation of an embodiment of the presentapparatus for determining the isoelectric pH.

FIG. 2 is a graph of the measured potential difference as a function ofpH for Cu₂Sb in a 1 mM KCl solution using the apparatus of FIG. 1.

FIG. 3 is a graph of the capacity retention in percent as a function ofthe number of charging and discharging cycles for a Cu₂Sb anode andLiCoO₂ cathode in 1M LiClO₄ in a solution of ethyl carbonate, dimethylcarbonate and diethyl carbonate in a 1:1:1 ratio, for coated anduncoated Cu₂Sb.

FIG. 4 is a schematic representation of an embodiment of anelectrophoretic apparatus used to increase the deposited coatingthickness.

FIG. 5 is a graph of the current as a function of the applied potential,showing cyclic voltamograms for Cu₂Sb in an AlPO₄ deposition solution.

FIG. 6 shows a survey X-Ray Photoelectron Spectrum (XPS) forelectroplated Cu₂Sb, and high-resolution spectra for the copper 2p,phosphorous 2p, and aluminum 2p regions.

FIG. 7 shows a survey XPS spectrum of a coating formed with a positiveapplied potential after the self-assembly process had been completed,and high-resolution spectra for the copper 2p, phosphorous 2p, andaluminum 2p regions for electroplated Cu₂Sb having a self-assembledAlPO₄ coating.

FIG. 8 shows a survey XPS spectrum of a coating formed with a negativeapplied potential after the self-assembly process had been completed,and high-resolution spectra for the copper 2p, phosphorous 2p, andaluminum 2p regions for electroplated Cu₂Sb having a self-assembledAlPO₄ coating.

DETAILED DESCRIPTION OF THE INVENTION

Briefly, embodiments of the present invention include an apparatus andmethod for measuring the isoelectric pH for materials deposited on orotherwise affixed to and in contact with an electrode surface. Anotherembodiment of the invention is to provide a method for utilizing theisoelectric pH to form nanometer thickness, self-assembled layers onsuch materials. If the self-assembled thickness proves to beinsufficient, an apparatus and method is provide for increasing thelayer thickness by utilizing electrochemical methods in cooperation withself-assembly to obtain the desired properties.

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In the FIGURES, similar structure will be identified usingidentical reference characters. Turning now to FIG. 1, a schematicrepresentation of an embodiment of the present apparatus, 10, fordetermining the isoelectric pH is illustrated. Lengths of about 1 mm, 12a, 12 b, of silver wire reference electrodes, 14 a, 14 b, are coatedwith AgCl and exposed to aqueous solution, 16, contained in cylindricalchamber, 18, having rectangular secondary chamber, 20, in fluidcommunication therewith. Clearly, other chamber geometries may beemployed. Leak-proof, insulating material, 22 a, 22 b, covers silverwires 14 a, 14 b as these wires passes through walls, 24, and 26, ofchamber 18 and secondary chamber 20. Material to be coated, 28, isaffixed to circular disk, 30, mounted on shaft, 31, which is supportedand turned by motor, 32, controlled by motor controller, 34. Means, 35,for measuring the pH of solution 16 are provided. Voltmeter orpotentiometer, 36, measures the potential difference between AgCl-coatedsilver wires 14 a and 14 b, which is recorded on data processor, 38,which also controls motor controller 34.

Since the isoelectric pH is not a function of the material morphology orfinal geometric configuration, the use of a planar electrode surface inthe apparatus is advantageous. Chamber 18 is filled with a solutioncontaining a supporting electrolyte, for example, potassium chloride(KCl). Generally, the solvent will be water, as pH is most applicable toaqueous solutions. However, other solvents, or combinations of solvents,may be used, including aqueous solutions with other water-misciblesolvents. Electrolyte solutions not containing water may also beemployed where a supporting electrolyte has sufficient solubility toproduce adequate conductivity for the electrolyte system; that is, thesupporting electrolyte concentration must be sufficiently high that thesolution resistance does not affect the potential readings of the tworeference electrodes 14 a and 14 b. Electrolyte concentration must besufficiently low such that when reference electrode 14 b is placed inthe vicinity of material to be coated 28, the aggregation of chargedspecies in solution (often referred to as the diffuse or boundarylayer), having opposite charge to that of the surface of material 28,can be detected. Any electrolyte species that, when dissolved, meetsthese criteria may be used. The concentration of the electrolyte isgenerally between 0.001 and 100 mM. It should be noted, however, thatthe electrolyte concentration chosen, even within this range, willaffect the ability to detect the aggregation of charged species near thesurface of the material of interest.

Reference electrode 14 b is disposed in the center of chamber 18. Whileother reference electrodes may be used, an Ag wire having anelectrodeposited AgCl film was used for the present EXAMPLE. Insulatingjacket 22 b surrounding Ag wire 14 b assures that only a small portion12 b (approximately 1 mm) of reference electrode 14 b is exposed to theliquid containing the supporting electrolyte 16. The area of theelectroactive surface of material to be coated 28 exposed to solution 16is chosen to be sufficiently large as to minimize edge effects. Aminimum area is on the order of approximately 1 cm² for a circular disk.The maximum diameter of the electrode structure (disk plus material tobe coated) was chosen to be smaller than the diameter of the inner wallsurface of the main chamber by approximately 20%, assuming a cylindricalchamber, to eliminate unwanted effects from solution interaction withthe chamber walls. While the gap between rotating disk 30 and the mainchamber inner wall surface can be made larger, a substantially smallergap may adversely affect the measurements. The material to be coated 28may have a diameter of about 5 cm.

Once the disk has been prepared, it is attached to shaft 31 which may berotated by motor 32 at speeds between zero and tens of thousands of rpm.Faster spin rates facilitate the measurements as a larger potential dropis induced between the reference electrodes as the spin rate isincreased. The disk/shaft assembly is placed in chamber 18 assuring thatthe center of disk 30 is directly over reference electrode 14 b at adistance≦1 mm, the reference electrode being placed as close as possibleto spinning disk 30 such that the aggregation of charged species onmaterial surface 28, which will be different than the equilibriumdistribution in the bulk fluid, induces a potential difference betweenthe two reference electrodes which can readily be detected. Thereference electrode need not penetrate the diffuse layer, however.

Secondary chamber 20 in fluid communication with chamber 18 throughtube, 39, permits solution 16 from chamber 18 to enter and leave,contains second reference electrode 14 a. Openings in wall 26, not shownin FIG. 1, permit pressure equalization between chamber 18 and secondarychamber 20, thereby facilitating fluid movement between the twochambers. The secondary chamber isolates reference electrode 14 a fromthe convective mass transfer of ions dissolved in solution 16 that isinduced by the spinning disk assembly. Reference electrode 14 a may havethe same construction as the reference electrode 14 b, ensuring thatthere is approximately zero potential difference between the electrodesbefore disk 30 is placed in solution. However, reference electrode 14 amay be based on any redox chemistry for which the potential differencebetween that electrode and the reference electrode 14 b is known(hereinafter referred to as the “resting potential difference”), andvice versa, as long as both reference electrodes act identically withrespect to voltage changes in the spatial location of charged species insolution. In practice, less than about 0.1 mV resting potentialdifference is advantageous.

To determine the isoelectric pH of material 18, using apparatus 10 ofFIG. 1 hereof, electrolyte solutions are prepared at different pHvalues. The disk assembly was spun at a chosen angular velocity in asolution with a selected pH and the potential difference betweenreference electrodes 14 a and 14 b was measured using a digitalmultimeter, as an example of an effective potential differencemeasurement apparatus. In general, higher spin rates provide betterresults, but rates>0 rpm to about 10,000 rpm are effective. Since themeasured potential difference is dependent on the spin rate,measurements may be taken as a function of spin rate at a given pH toimprove the accuracy of the data. Such measurements are repeated forsolutions having different pH values until the measured potentialdifference between the reference electrodes is equal to zero or equal tothe resting potential difference for reference electrodes havingdissimilar redox potentials. This is the isoelectric pH of the material.A single solution where the pH is adjusted by small additions of acid,base, buffer, etc. between measurements, may also be used.

Returning to apparatus 10 of FIG. 1, as stated hereinabove, referenceelectrode 14 a disposed in secondary chamber 20 is separated fromsolution 16 in chamber 18 by tube 39 such that the ions in solution fromboth chambers can communicate without the influence of convective masstransfer. This has resulted in measured potential differences an orderof magnitude higher than what is typically reported by Paul J. Sides andJames D. Hoggard in “Measurement of the Zeta Potential of Planar SolidSurfaces by Means of a Rotating Disk,” Langmuir 2004, 20, 11493-11498.As an example, Sides and Hoggard reports differences on the order of 0.1my; whereas, differences on the order of a millivolt (FIG. 5 of Sidesand Hoggard) are observed by Sides and Hoggard in only the mostadvantageous conditions, and for materials with high zeta potentials.Potential differences measured using apparatus 10 of FIG. 1 hereof aretypically on the order of mV, as may be observed in FIG. 2, hereof,which is a graph of the measured potential difference as a function ofthe pH of the solution for Cu₂Sb as material 28. The typical measuredpotential differences between the two reference electrodes reported bySide and Hoggard are difficult to distinguish from noise; thus, theembodiment described herein alleviates the most severe constraint ofSides and Hoggard, that the measurements are recorded at solutionconcentrations of less than about 1 mM. These solutions generate lessthan a 0.1 mV difference between the reference electrodes, which isdifficult to quantitatively distinguish from noise for suchmeasurements.

Another embodiment of the present invention is to use the measuredisoelectric pH for the deposition of an electrolyte separation layer.Once the isoelectric pH for a material has been determined by theabove-described procedure, or obtained from other sources thereof,solid-state ceramics which have shown to be electrically insulating butionically conducting at nanoscale thicknesses may be self-assembled ontoboth anode and cathode materials. Example materials are AlPO₄, Al₂O₃,ZnO, and Bi₂O₃. AlPO₄ has been shown by other investigators to functionas a solid-state electrolyte for lithium-ion batteries. Self-assemblymethodology includes: (1) dissolving the appropriate precursors for thesolid-state electrolyte in an aqueous solution; (2) adjusting thesolution pH so that it is between the isoelectric pH of the electrodematerial, and that for the desired solid-state electrolyte, for whichthe isoelectric pH values are available in the literature; and (3)contacting the electrode material which can now be of any morphology andin any geometric configuration with the solution. The driving force forthe self-assembly is electrostatics, where the overall net charge on thesurface of the electrode material is opposite to the charge on thesurface of the solid-state electrolyte. The above-mentioned ceramic andoxide materials can exist in solution as a colloidal suspension, and thesurface charge of the material is controlled by the solution pH.Selection of the solution pH approximately midway between theisoelectric pH electrode material and that of the desired solid-stateelectrolyte material (coating material) generates coatings having thestrongest binding to the substrate (the electrode material). For someapplications, it may be useful to select a pH other than midway betweenthe isoelectric pH of the two materials in order to vary the thicknessof the film, despite the loss of some adhesion strength of the coatingto the electrode. Generally however, the solution pH should be somewherebetween the respective isoelectric pH values. Coatings typically form inapproximately 30 min., although shorter times are possible ifelectrostatic equilibrium has been reached.

Electrostatic formation of a solid electrolyte coating is advantageous.First, the growth of the coating is self-limiting because once thesurface charge has been neutralized there is no longer a driving forcefor the solid electrolyte coating thickness to increase. Anticipatedthicknesses for the resulting coatings using this methodology arebetween about 1 and 20 nm (typically, on the order of 5 nm), and can besomewhat adjusted by choosing the pH and/or applied potential (as willbe discussed hereinbelow). Second, uniform coatings without pinholedefects will be formed because a local driving force for assembly willexist if any bare electrode material is exposed to the solution. Theresults of self-assembled AlPO₄ deposited using the apparatus and methoddisclosed hereinabove demonstrate that uniform coatings havingmechanical stability can be formed at ambient pressure and temperaturefrom aqueous solutions. This apparatus and procedure can be used to coata broad range of materials with varying morphologies and surface areaswith coatings that can be tailored to provide the mechanical,electrical, and/or ionic conductivity properties of interest.

As will be set forth in detail in the EXAMPLES hereinbelow, if thethickness obtained from self-assembly is insufficient to stop electrontunneling or is otherwise insufficiently insulating, a slightoverpotential (generally in the range between about one millivolt andabout one volt, but typically between 10 and 100 mV) can be applied toincrease the thickness of the coating through an electrodepositionprocedure. Chronoamperometry or a pulsed chronoamperometry is expectedto initiate an electrophoretic mechanism for the above-mentionedceramics. Other electrodeposition techniques may also prove to beeffective.

Having generally described embodiments of the present invention, thefollowing example provides additional details.

Example 1

Using electrodeposition, Cu₂Sb (an anode material for lithium-ionbatteries) was deposited on a copper circular disk with a diameter ofapproximately 5 cm in accordance with J. M. Mosby and A. L. Prieto,Direct Deposition of Cu₂Sb for Lithium-Ion Battery Anodes,” J. Am. Chem.Soc. 2008, 130, 10656-10661. A stainless steel shaft was connected tothe disk and installed in apparatus 10 of FIG. 1, hereof. Theisoelectric pH of the Cu₂Sb was determined using 1 mM KCl as thesupporting electrolyte, Ag/AgCl as the reference electrodes, and a spinrate of approximately 750 rpm. The pH of the solution was controlled byadding either dilute hydrochloric acid to lower the pH or diluteammonium hydroxide to increase the pH. Other reagents may be employed toadjust the solution pH. The isoelectric pH of Cu₂Sb was determined to beapproximately 7.5 (FIG. 2, hereof).

The reported isoelectric pH reported for AlPO₄ is 4.7 (See, e.g., J. Liuand A. Manthiram, Understanding the Improvement in the ElectrochemicalProperties of Surface Modified 5 V LiMn_(1.42)Ni_(0.42)Co_(0.16)O₄Spinel Cathodes in Lithium-Ion Cells,” Chem. Mater. 2009 21, 1695-1707).Therefore, the pH of the electrodeposition solution is about 6.1, themidpoint between the isoelectric pH of AlPO₄ and Cu₂Sb. This solution pHassures the maximum magnitude of opposite surface charges on the AlPO₄and Cu₂Sb to promote self-assembly since, as the solution pH becomesincreasingly positive, that is, more basic, when compared to theisoelectric pH, the magnitude of the net surface charge becomesincreasingly negative. Conversely, when the solution pH becomesincreasingly negative, that is, more acidic, the magnitude of the netsurface charge becomes increasingly positive.

AlPO₄ was coated onto Cu₂Sb thin films that had been electroplated ontocopper substrates by self-assembly in a solution described in EXAMPLE 2,hereof including approximately 2 mM of NH₄H₂PO₄ (ammonium phosphatemonobasic), 1.8 mM of Al(NO₃)₃. 9 H₂O (aluminum nitrate nonahydrate),and the pH was adjusted to 6.1 using NH₄OH (ammonium hydroxide). Theconcentrations of the two precursors were in an approximately 1:1 molarratio of NH₄H₂PO₄ to Al(NO₃)₃. 9 H₂O, and in millimolar concentrations,in order to avoid agglomeration of the colloids once the pH is adjusted.

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy(SEM) coupled with energy dispersive spectroscopy (EDS) techniques wereused to confirm that AlPO₄ self assembly onto the Cu₂Sb had occurred.Although XPS peaks from Al and P confirm that AlPO₄ had self-assembled,Sb and Cu XPS peaks were observed to also be present, indicating a thinAlPO₄ layer. Negative electrodes having bare Cu₂Sb, and Cu₂Sb coatedwith self-assembled AlPO₄ were tested in full cells with a LiCoO₂cathode based positive electrode in a liquid electrolyte consisting of1M LiClO₄ in ethylene carbonate (EC), dimethyl carbonate (DMC) anddiethyl carbonate (DEC) in a (1:1:1) ratio by volume. The negativeelectrode modified with AlPO₄ showed much better capacity retention thanthe bare negative electrode during cycling as shown in FIG. 3,indicating that self-assembled AlPO₄ provides mechanical stabilityduring cycling, thereby reducing the capacity loss that is found frommaterial pulverization and material loss from volume expansion and otherelectrochemical processes associated with cycles of charging anddischarging. The solid squares and circles of FIG. 3 show the charge anddischarge capacity retention of the cell with the negative electrodecomprising Cu₂Sb covered with self-assembled AlPO₄, respectively, whilethe open boxes and circles show the charge and discharge capacityretention for cells having bare Cu₂Sb anodes, respectively. Cells werecharged and discharged between 2.75 and 3.35 V.

To ensure that the AlPO₄ coating was robust, SEM images and EDS spectrawere collected after electrochemical cycling. SEM images of uncoatedCu₂Sb films electrodeposited onto a copper substrate beforeelectrochemical cycling showed the cubic morphology of the Cu₂Sb, whileSEM images of a Cu₂Sb electrodeposited film coated with AlPO₄ byself-assembly clearly showed the AlPO₄ coating. To establish thepresence of the aluminum and phosphorous in the coated Cu₂Sb, and theabsence of these elements in the uncoated Cu₂Sb, EDS spectra of therespective films were collected. EDS spectra of the uncoated Cu₂Sbconfirmed the presence only of copper and antimony with trace amounts ofoxygen and carbon due to a thin oxide layer, and graphitic carbon on theCu₂Sb surface, respectively. The EDS spectra for the AlPO₄ coated Cu₂Sbverified the presence of aluminum and phosphorous in addition to theelements identified for the uncoated film. The observed presence ofchlorine is due to residual lithium perchlorate, LiOCl₄, from theelectrochemical cycling experiments.

Example 2

The combination of the self-assembly procedure described hereinabovewith electrodeposition to increase the coating thickness, is nowdescribed with a Cu₂Sb substrate coated with AlPO₄ as an example.

In order to increase the solid electrolyte coating thickness using anelectrodeposition procedure, the bare electrode material is physicallyattached to a current collector, such as copper foil. The self-assembledcoating is then added using the above-described procedure. Onceself-assembly is complete, a counter and reference electrode may beplaced in the solution and the electrode material attached to thecurrent collector is made the working electrode. A potential is thenapplied to the working of the correct polarity with respect to the opencircuit potential. The potential polarity will be specific to theelectrode and solid electrolyte material, and is applied until thecurrent decays to a desired value, most often approximately zero. Themagnitude and polarity of the applied potential compared to the opencircuit potential; the time for which the potential is applied; and thepH of the solution are three independent variables that will determinethe rate at which the solid electrolyte coating will be deposited, andthe final thickness of the coating. While one may conductelectrodeposition at the pH for which the self-assembly occurs, other pHvalues may be employed.

FIG. 4 is a schematic representation of an embodiment of theelectrophoretic deposition apparatus, 40, used to increase the AlPO₄coating thickness. Electrophoretic deposition implies that chargedparticles (such as for a suspension of colloidal particles having a netsurface charge) are moving in response to an applied external electricfield, while electrodeposition implies that current is being passed toinitiate a chemical reaction from an electrode resulting in a depositionof material onto the electrode. An AlPO₄ deposition solution, 42, isplaced in deposition chamber, 44, containing working electrode, 46(Cu₂Sb that has been electrodeposited onto a copper substrate, in thisexample), counter electrode, 48 (platinum in this case, but othermaterials such as stainless steel may also be used), and referenceelectrode, 50 (sodium saturated calomel electrode (SSCE), as anexample). Potentiostat/galvanostat, 52, applies voltage between workingelectrode 46 and counter electrode 48, and measures the current flowingtherebetween. Potentiostat/galvanostat, 52 also measures the voltagebetween working electrode 46 and reference electrode 50.

The placement of the electrodes and their spatial relationship to eachother may affect the deposition process. To determine the potentialrange, with respect to the open circuit potential (OCP), cyclicvoltamograms of Cu₂Sb in AlPO₄ deposition solution were plotted in FIG.5. The Cu₂Sb was electroplated onto a copper current collector having asurface area of 0.32 cm². The OCP is defined as the potential at whichno current passes through the cell. For this system, a potential windowof approximately 75 mV exists as illustrated by the vertical dashedlines. The electrodeposition solution was the same as that for theself-assembly solution, but supporting electrolytes may be addedfacilitate transfer of charged species. However, supporting electrolytesare chosen such that they do not undergo redox reactions or otherdetrimental processes under the electrodeposition conditions used toincrease the coating thickness, thereby adversely affecting thedeposition.

AlPO₄ was coated onto Cu₂Sb thin films that had been electroplated ontocopper substrates by self-assembly in the solution set forth in EXAMPLE1, hereof. Once the self-assembly process is completed,electrodeposition may be used to increase the AlPO₄ coating thickness.In this example, electrophoretic deposition is appropriate, althoughother electrodeposition techniques may be utilized. Two conditions werechosen: (a) 30 mV more positive than the OCP; (b) 30 mV more negativethan the OCP. These potentials were chosen because they both are withinthe range determined from the cyclic voltamograms contained in FIG. 5.Additionally, the more positive potential is expected to induce apositive surface charge on the Cu₂Sb whereas the negative potential anegative surface charge. To compare the effect of applying a positiveand negative potential when compared to the OCP, X-Ray PhotoelectronSpectroscopy (XPS) was used since it is a quantitative surface sensitivecharacterization technique.

FIG. 6 shows XPS spectra of electroplated Cu₂Sb. The survey spectrum andhigh resolution spectra do not show the presence of aluminum orphosphorous, which confirms that an AlPO₄ coating is not present. Notethat the region of the phosphorous 2p and aluminum 2p peaks are in closeproximity to copper 3s and copper 3p peaks, respectively. FIG. 7 showsXPS spectra of a coating formed with a positive applied potential afterthe self-assembly process had been completed. A clear difference betweenthe spectra in FIGS. 6 and 7 may be observed. In the phosphorous 2pregion of FIG. 7, a peak is present, while for FIG. 6 a peak in the sameregion is not observed. Additionally, this peak may be located in thesurvey spectrum in close proximately to the copper 3s peak; thephosphorous 2p peak is equivalent in magnitude to the copper 3s peak.When analyzing the aluminum 2p region in FIG. 7, the peak shape haschanged due to the presence of the aluminum 2p peak when compared to thesame region plotted in FIG. 6.

FIG. 8 shows XPS spectra of a coating formed with a negative appliedpotential after the self-assembly process had been completed. Byobserving the phosphorous 2p region, these XPS spectra are moreindicative of an AlPO₄ coating that has been self-assembled onto thesurface of Cu₂Sb without the addition of an applied potential. While apeak is observed in both the high-resolution and survey spectra, thepeak is approximately half the magnitude of the copper 3s peak. Applyinga negative potential when compared to the OCP, therefore, does notincrease the coating thickness when compared to a coating that has beenself-assembled using the procedures described hereinabove. This islikely the result of the application of a negative potential inducing anegative surface charge. Since the net surface charge of the AlPO₄ insolution is also negative, there is insufficient driving force topromote the assembly of additional AlPO₄ onto the Cu₂Sb surface toincrease the coating thickness. By contrast, applying a positivepotential when compared to the OCP results in a positive surface chargewhich produces in an increase in the AlPO₄ coating thickness. This isconfirmed by the phosphorous 2p peak in FIG. 7 which is approximatelytwice the magnitude of the peak contained in FIG. 8 when using thecopper 3s peak as a reference. Since XPS is a quantitative surfacesensitive technique, the increase in the phosphorous 2p peak magnitudewhen compared to the copper 3s peak is indicative of an increase in thesurface AlPO₄ coating thickness.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The embodiments were chosen and described in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

1. An apparatus for determining the isoelectric pH of a material,comprising in combination: a first chamber adapted for containing asolution having a chosen pH comprising: a circular disk having an axis,a first face, and an opposing second face onto which said material isdisposed; a shaft attached to the first face of said disk; a firstreference electrode disposed in the vicinity of the second face of saiddisk and in the vicinity of the axis; and a hollow tube affixed to awall of said first chamber at an orifice therein; a second chamber influid communication with said first chamber through said tube,comprising a second reference electrode; means for rotating said shaftat a chosen rate; and means for measuring the potential differencebetween said first reference electrode and said second referenceelectrode.
 2. The apparatus of claim 1, further comprising means formeasuring the pH of the solution.
 3. The apparatus of claim 1, whereinsaid material is deposited onto the second face of said disk.
 4. Amethod for determining the isoelectric pH of a material, comprising thesteps of: affixing the material on one face of a circular disk having anaxis of rotation and disposed in a solution having a selected pH valuein a first chamber; rotating the disk at a chosen rate; measuring thepotential difference between a first reference electrode disposed in thevicinity of both the axis of the disk and the material and a secondreference electrode disposed in a second chamber in fluid communicationthrough a tube with said first chamber; adjusting the pH of the solutionsuch that the potential difference is approximately equal to the restingpotential difference between the first reference electrode and thesecond reference electrode; and measuring the pH of the solution whenthe potential difference is approximately equal to the resting potentialdifference.
 5. The method of claim 4, wherein the material comprisesCu₂Sb.
 6. The method of claim 5, wherein said step of affixing the Cu₂Sbon one face of a circular disk is achieved by deposition.
 7. The methodof claim 6, wherein the deposition comprises electrodeposition.
 8. Themethod of claim 5, wherein the disk is rotated at approximately 750 rpm.9. The method of claim 4, wherein the solution comprises aqueous KCl.10. The method of claim 4, wherein said step of adjusting the pH of thesolution is achieved using hydrochloric acid to lower the pH andammonium hydroxide to raise the pH of the solution.
 11. A method fordepositing a coating material onto a substrate, comprising the steps of:determining the isoelectric pH of the substrate; determining theisoelectric pH of the coating material; preparing a solution of thecoating material having a pH between the isoelectric pH of the substrateand the isoelectric pH of the coating material; and immersing thesubstrate into the solution for a time sufficient for self-assembly ofthe coating material onto the substrate.
 12. The method of claim 11,wherein the pH of the solution is approximately the average of theisoelectric pH of the substrate and the isoelectric pH of the coatingmaterial.
 13. The method of claim 11, wherein said substrate comprisesCu₂Sb.
 14. The method of claim 11, wherein said coating material ischosen from AlPO₄, Al₂O₃, ZnO, and Bi₂O₃.
 15. The method of claim 11,further comprising the step of increasing the thickness of the coatingmaterial on the substrate.
 16. The method of claim 15, wherein said stepof increasing the thickness of the coating material compriseselectrodeposition of additional coating material onto the self-assembledcoating material.
 17. The method of claim 16, wherein saidelectrodeposition comprises electrophoretic deposition.
 18. The methodof claim 17, wherein said substrate comprises Cu₂Sb.
 19. The method ofclaim 18, wherein said Cu₂Sb is electroplated onto a copper currentcollector.
 20. The method of claim 15, wherein said coating material ischosen from AlPO₄, Al₂O₃, ZnO, and Bi₂O₃.
 21. The method of claim 17,wherein said substrate comprises Cu₂Sb, said self-assembled coatingcomprises AlPO₄, and the electrophoretic deposition of the AlPO₄ ontothe self-assembled coating of AlPO₄ on said Cu₂Sb substrate is performedwith the self-assembled coating at a positive voltage relative to opencircuit potential.
 22. The method of claim 21, wherein the solution ofthe coating material for said electrophoretic deposition is the samesolution used for said self-assembly of said coating material.